Short-arc gas discharge lamp

ABSTRACT

A short-arc gas discharge lamp having a discharge vessel provided with a gas filling in which the discharge takes place, and with two electrodes of a high melting point metal facing each other. The end of at least one of the electrodes facing the discharge is conical and has a recess located on the axis of the cone, which recess accommodates a quantity of thorium metal. The recess extends to such a distance from the closed apex of the cone that during operation of the lamp transport of thorium metal to the apex of the cone is mainly effected by means of diffusion through the electrode material.

[ SHORT-ARC GAS DISCHARGE LAMP [7Sl Inventors: Tjepke Hendrik Ekkelboom; Wubbe Vriezc, both of Emmasingel. Eindhoven. Netherlands [73I Assignee: Philips Corporation New York. N.Y.

[22} Filed: Feb. 21, 1973 {21] Appl.N0.1334,466

[30) Foreign Application Priority Data Mar. l8 W72 Netherlands 7203663 I52] U.S. Cl. 313/217; BIB/54; 3l3/2I8; 3|3/346 DC [5|] Int. Cl H0lj 1/14; HUlj 1/28 [SKI Field of Search 3|3/54.2I7, 218, 346 DC. 313/346 R [56] References Cited lINlTED STATES PA'lliNTS ZXWTLW) l ll95 Anderson et ill 3 l V146 DC 1 Aug. 26, 1975 Schlcgel. 3|3/2l7 Glascock et al 313/346 D( 57 ABSTRACT A short-arc gas discharge lamp having a discharge vessel provided with a gas filling in which the discharge takes place, and with two electrodes of a high melting point metal facing each other. The end of at least one of the electrodes facing the discharge is conical and has a recess located on the axis of the cone. which re cess accommodates a quantity of thorium metal. The recess extends to such a distance from the closed apex of the cone that during operation of the lamp trans port of thorium metal to the apex of the cone is mainly effected by means of diffusion through the electrode material.

6 Claims, 2 Drawing Figures PATENTEB AUGZ 61975 SHORT-ARC GAS DISCHARGE LAMP The invention relates to a short-arc gas discharge lamp. that is to say. a discharge lamp in which the vapour pressure is higher than ll) atmospheres during operation and in which the distance between the elec trodes is shorter than the maximum diameter of the discharge space. The gas filling in which the discharge takes place consists in most cases mainly of mercury; it is alternatively possible for this gas filling to consist of a rare gas such as xenon. Furthermore the invention relates to an electrode for such a discharge lamp and to a method of manufacturing such an electrode.

The electrodes of short-arc gas discharge lamps generally consist of two cones whose apexes are arranged opposite each other and between which the arc is active during operation of the lamp. In a lamp for direct current operation the electrode operating as an anode may. however. alternatively be spherical. It is not necessary that the entire electrode is conical; electrodes consisting of a conical part and an adjoining, for example, cylindrical part will also be referred to hereinafter as conical electrodes. Electrodes having a conical end part in which the apex is rounded off, for example. spherically will likewise be considered as conical electrodes in the following description. Furthermore the expression conical electrode in this description also comprises an electrode which has a pyramid shape at the end facing the discharge. When switching on such a lamp an arc is produced between the electrodes, which are generally does not engage the apex of the cones but another area of the electrode. Due to heating of the electrodes as a result of the passage of current, the discharge path becomes shorter until upon reaching the operating temperature the arc terminates entirely at the apexes of the conical electrodes. A very small light transmitting surface emitting an intense light is then produced. Such lamps are therefore very suitable to be arranged in projection-optical systems.

Since in a discharge lamp of the kind described the electrodes are thermally loaded to a high extent during operation. only materials which are resistant to very high temperatures are suitable for these electrodes. The electrodes therefore generally consist of tungsten or of an alloy which mainly comprises tungsten. It is desirable that the electrode which serves as a cathode during operation of the lamp has a low work potential so that a satisfactory emission of electrons is obtained. If the lamp is intended for alternating voltage operation the two electrodes alternately serve as an anode and a cathode. In that case the two electrodes must of course have a low work potential. A step which is often used to decrease the work potential of an electrode is the use of the above-mentioned heat-resistant materials. particularly tungsten in a thoriated form. This means that the electrode material comprises a small quantity of thorium oxide. for example, I to 3'/( by weight. During operation of the lamp thorium oxide produces slight quantities of thorium which is a satisfactory electronemissive matcrial.

A drawback of the said electrodes of thoriatcd tung sten is that the lamps prm ided with these electrodes exhibit a shortage of emissive material at the electrode point after a given operation period (for example, after 51W operating hours). I his shortage is produced by a too low post-supply of thorium from the electrode ma tcrial. This is caused by a recrystallisation of the electrode point occuring at a high temperature so that the surface along the crystal grain boundaries available for diffusion of thorium is severely reduced. The shortage of emissive material at the electrode point results in the are becoming unstable. In fact the arc will terminate at an area on the surface of the cone where there is still a sufficient postsupply of thorium. At this area recrys tallisation will also occur so that the point of termination is displaced again. The said instability is very troublesome especially when the lamp is used for projection purposes.

A further drawback of the electrodes of thoriated tungsten is that the electrode can comprise only comparatively little thorium so that the quantity of thorium available for diffusion to the electrode point is quickly exhaustedv If the quantity of thorium which can be effectively active for the electron emission is exhausted, the temperature of the electrode will assume inadmissibly high values so that the electrode is deformed. Furthermore, a strong evaporation of tungsten will take place as a result of the then occurring very high electrode temperature so that a strong blackening of the wall of the discharge vessel is obtained. The said draw back might be obviated by not using thorium oxide but metallic thorium instead which is accommodated in a space in the electrode.

In order to obtain a large supply of electron emmissive material in electrodes for short-arc gas discharge lamps it is known to manufacture conical electrodes provided with a cavity accommodating electron emis sive material. particularly throrium oxide (see German Patent No. 733986). One or more bores extend from this cavity to the surface of the electrode Thorium may reach this surface through these bores. This known construction has the drawback that readily too much emissive material reaches the electrode surface from the supply cavity through the bores, evaporates at that surface and finally gives rise to an inadmissible blackening of the wall of the discharge vessel. When using metallic thorium as an emissive material in the known construction the thorium supply in the electrode will already have disappeared after several operating hours and the lamp will be severely blackened because thorium has a relatively high vapour pressure at the operating temperature of the electrode.

A conical electrode for short-arc gas discharge lamps is known from United Kingdom Patent Specification 864,234 in which the cone is provided with a wide, open slot. The purpose of the slot is to obtain sharp edges on the electrode so that the discharge can be easily initiated by means of an auxiliary discharge between the electrode and an auxiliary electrode arranged in the vicinity thereof. It has been stated that the surface of the slot may be coated with a layer of thorium as an emissive material. It has been found that this electrode has the drawback of an unstable discharge because the electrode has two points at its apex at which the discharge may alternately terminate. Furthermore the electrode has the drawback that thorium if used. evap orates quickly and gives rise to blackening of the discharge vessel An object of the invention is to provide a short-arc gas discharge lamp having an electrode which for the purpose of reducing the work potential is pro\ ided with a quantity ol'elcctron emissive material and with which a stable discharge is obtained during the lifetime of the lamp.

According to the invention a short-arc gas discharge lamp has a discharge vessel provided with a gas filling in which the discharge takes place and with two electrodes of a high meltingpoint metal facing each other. the end of at least one of the electrodes facing the discharge being conical and having a recess located on the axis of the cone. which recess accommodates a quantity of thorium metal and is characterized in that the recess extends axially to such a distance from the closed cone apex that during operation of the lamp transport of thorium metal to the cone apex mainly takes place by means of diffusion through the electrode material.

The invention is based on the recognition of the fact that a stable operating lamp can be obtained if a conical electrode is used which comprises metallic thorium and in which the diffusion of thorium is mainly guided to the electrode point. Thorium is in that case to be shielded sufficiently. at least in the vicinity of the electrode point so that an inadmissibly strong evaporation of thorium is prevented. In a lamp according to the in vention the electrode has a supply of thorium which is sufficiently large to provide the electrode point with the satisfactory electron emissivc thorium during the entire lifetime. The supply of thorium to the electrode point during operation of the lamp is effected along the crystal boundaries of the electrode material. This supply is predominantly guided into the direction of the electrode point so that the lamp exhibits a stable discharge during its entire lifetime. The apex of the electrode in a lamp according to the invention is closed so that the discharge cannot terminate at the thorium supply itself. As a result an unwanted large evaporation of thorium is prevented. The recess accommodating thorium need not be completely shut off from the discharge space. This recess may be in communication with the discharge space at areas which are farther remote from the cone apex where the temperature is considerably lower than the temperature at the cone apex during operation of the lamp. In that case it is possible that still some diffusion of thorium occurs along the cone surface.

The recess in an electrode of a lamp according to the invention preferably has the shape of a narrow slot whose section at right angles to the axis of the cone is substantially rectangular. This slot is in communication with the cone surface but is closed at the cone apex by a pressure-welded joint. This welded joint provides a satisfactory shielding of the thorium supply from the discharge arc. The electrode has the advantage that as a result of the welded joint crystal boundaries present in the electrode material or being produced by recrystallisation during operation have a preferred direction in the direction of the electrode apex. As a result the diffusion of thorium substantially extends exclusively into the direction of the electrode apex. Furthermore this electrode has the advantage that it can be satisfac torily degassed during manufacture of the lamp because the slot and the thorium metal present therein are in communication with the discharge space.

In an advantageous embodiment of a lamp according to the invention the electrode has a slot as described above in which the width of the slot increases as the distance from the cone apex increases. The maximum width of the slot is not less than 0.05 mm and not more than 0.25 times the diameter of the cone base and the depth of the slot has a value of between 4 to 50 times the maximum width. The maximum width of the slot is preferably not more than 0.10 times the diameter of the cone base and the depth of the slot has a value of between l5 and 20 times the maximum width.

Tungsten or an alloy mainly comprising tungsten is preferably chosen as a material for the electrode in a lamp according to the invention because these materials are resistant to high temperatures. The said materials may alternatively be used in a thoriated form.

An electrode according to the invention is preferably manufactured by a method according to the invention in which one end of a cylindrical rod or wire of high melting point metal (for example, tungsten) is provided with a narrow rectangular slot. for example. by grinding. This slot which is located on the axis of the rod or wire extends radially and axially up to the outer surface of the rod or the wire. The slot is subsequently entirely or partly filled with metallic thorium, for example, in the form of a thorium plate. Subsequently. the slot is closed by means of a pressurewelded joint, for example, a spot weld at the end of the rod or wire. Finally. the end is given the shape of a cone having the desired apex and a rounding. This may be effected, for example. by means of a grinding operation.

The invention will now be described in greater detail with reference to a drawing. In the drawing FIG. 1 shows an embodiment of a short-arc gas discharge lamp according to the invention and FIG. 2 shows the electrode operating as a cathode of FIG. I on a larger scale.

The lamp of FIG. I is a short-arc xenon discharge lamp which requires a power of approximately L600 Watts during operation. The lamp is formed as a direct current lamp and its xenon-filled quartz glass discharge vessel I includes an anode body 2 of tungsten which is secured to the tungsten current supply conductor 3 passed to the exterior in a vacuum tight manner. The cathode is denoted by 4 and consists of a cylindrical rod having a diameter of 3 mms. The rod 4 consists of tungsten which comprises a few percent of thorium oxide. The electrode 4 has a conical end 5 and is provided with a slot 6 according to the invention. This slot 6 is closed at the apex and comprises a supply of thorium metal.

In FIG. 2 the cathode of the lamp of FIG. I is shown in a cross-section on a large scale. The cylindrical cathode ll of thoriated tungsten has a conical end part 12. The end of the cathode has a recess 14, 15 obtained by grinding a rectangular slot. This slot is partly filled up with a plate of thorium IS. The slot is closed at the end by means of a spot-welded joint 16. By making the spot weld the slot has acquired a width which increases as the distance from the apex increases. The electrode apex 13 is provided with thorium metal during operation of the lamp. which metal comes from the supply of thorium 15 by means of diffusion mainly along crystal faces. for example. along the spot-welded surface 16, extending into the direction of the apex 13. The cone base of the cathode I 1 has a diameter of 3 mm. The slot l4, 15 has a depth of approximately 3 mm and a maximum width of approximately 0. [5 mm.

A lamp having a construction as shown in FIGS. I and 2 is found to have a stable discharge during its entire lifetime (approximately 2.000 operating hours).

What is claimed is:

l. A short-arc gas discharge lamp comprising a discharge vessel provided with a gas filling in which the discharge takes place and with two electrodes of a high melting point metal facing each other. the end of at least one of the electrodes facing the discharge being conical and having a narrow recess located on the axis of the cone, the apex of said cone being closed. said recess accommodating a quantity of thorium metal. said recess extending axially from the closed cone a distance such that during operation of the lamp transport of thorium metal to the cone apex is mainly effected by means of difi'usion through the electrode material said recess has the shape of a narrow slot ofwhich a section at right angles to the axis of the cone is substantially rectangular, which slot is in communication with the surface of the cone and is closed at the apex of the cone by means of a pressure-welded joint,

2. A short-arc gas discharge lamp as claimed in claim I, wherein the width of the slot increases as the dis tance from the apex of the cone increases, the maxi mum width of the slot being not less than 0.05 mm and not more than 0.25 times the diameter of the base of the cone and the depth of the slot being equal to 4 to 50 times the maximum width.

3. A short-arc gas discharge lamp as claimed in claim 2, wherein the maximum width of the slot is not more than 0.10 times the diameter of the base of the cone and that the depth ofthe slot is equal to l5 to times the maximum width.

4. A short-arc discharge lamp as claimed in claim I, wherein the conical end of the electrode consists of a material selected from the group consisting of tungsten, a tungsten alloy, thoriated tungsten and a thoriated tungsten alloy,

5. An electrode for use in a short-arc gas discharge lamp comprising a conical end and having a narrow recess located on the axis of the cone. the apex of said cone being closed, a quantity of thorium metal being disposed in said recess, said recess extending axially up to a predetermined distance from the closed cone apex such that during operation of the lamp transport of thorium metal to the cone apex is mainly effected by means of diffusion through the electrode material, the recess having the shape of a narrow slot of which a section at right angles to the axis of the cone is substantially rectangular, said slot being in communication with the surface of the cone and being closed at the apex of the cone by means of a pressure-welded joint.

6. A short-arc gas discharge lamp comprising a discharge vessel provided with a gas filling in which the discharge takes place and with two electrodes of a high melting point metal facing each other, the end of at least one of the electrodes facing the discharge being conical and having a narrow recess located on the axis of the cone, the apex of said cone being closed, said recess accommodating a quantity of thorium metal, said recess extending axially from the closed cone a distance such that during operation of the lamp transport of thorium metal to the cone apex is mainly effected by means of diffusion through the electrode material said recess has the shape of a narrow slot of which a section at right angles to the axis of the cone is substantially rectangular, which slot is in communication with the surface of the cone.

* k IIK 

1. A short-arc gas discharge lamp comprising a discharge vessel provided with a gas filling in which the discharge takes place and with two electrodes of a high melting point metal facing each other, the end of at least one of the electrodes facing the discharge being conical and having a narrow recess located on the axis of the cone, the apex of said cone being closed, said recess accommodating a quantity of thorium metal, said recess extending axially from the closed cone a distance such that during operation of the lamp transport of thorium metal to the cone apex is mainly effected by means of diffusion through the electrode material said recess has the shape of a narrow slot of which a sectioN at right angles to the axis of the cone is substantially rectangular, which slot is in communication with the surface of the cone and is closed at the apex of the cone by means of a pressure-welded joint.
 2. A short-arc gas discharge lamp as claimed in claim 1, wherein the width of the slot increases as the distance from the apex of the cone increases, the maximum width of the slot being not less than 0.05 mm and not more than 0.25 times the diameter of the base of the cone and the depth of the slot being equal to 4 to 50 times the maximum width.
 3. A short-arc gas discharge lamp as claimed in claim 2, wherein the maximum width of the slot is not more than 0.10 times the diameter of the base of the cone and that the depth of the slot is equal to 15 to 25 times the maximum width.
 4. A short-arc discharge lamp as claimed in claim 1, wherein the conical end of the electrode consists of a material selected from the group consisting of tungsten, a tungsten alloy, thoriated tungsten and a thoriated tungsten alloy.
 5. An electrode for use in a short-arc gas discharge lamp comprising a conical end and having a narrow recess located on the axis of the cone, the apex of said cone being closed, a quantity of thorium metal being disposed in said recess, said recess extending axially up to a predetermined distance from the closed cone apex such that during operation of the lamp transport of thorium metal to the cone apex is mainly effected by means of diffusion through the electrode material, the recess having the shape of a narrow slot of which a section at right angles to the axis of the cone is substantially rectangular, said slot being in communication with the surface of the cone and being closed at the apex of the cone by means of a pressure-welded joint.
 6. A short-arc gas discharge lamp comprising a discharge vessel provided with a gas filling in which the discharge takes place and with two electrodes of a high melting point metal facing each other, the end of at least one of the electrodes facing the discharge being conical and having a narrow recess located on the axis of the cone, the apex of said cone being closed, said recess accommodating a quantity of thorium metal, said recess extending axially from the closed cone a distance such that during operation of the lamp transport of thorium metal to the cone apex is mainly effected by means of diffusion through the electrode material said recess has the shape of a narrow slot of which a section at right angles to the axis of the cone is substantially rectangular, which slot is in communication with the surface of the cone. 